1. Field of the Invention
The present invention relates to the art of powder metallurgy, and more particularly to a method of manufacturing helical gears by compacting powdered materials which can prevent cracks or the like from being formed during the compacting process.
2. Description of the Prior Art
Hitherto, as an art of manufacturing helical gears as sintered products by compacting powdered materials, for example, there has been known the technology as disclosed in Japanese Patent Unexamined Publication No.H11-58087.
According to this technology, upper and lower punches respectively provided with outer circumferential helical teeth are arranged to engage with a floating die (an intermediary die) which is provided with inner circumferential helical teeth, wherein powdered materials are filled between the upper and lower punches. The upper punch is now moved downward to apply pressure to allow the powdered materials to be compacted. Then, the floating die is moved downward while holding the compacted product, with a predetermined holding power, between the upper and lower punches and is finally released from the compacted product.
In order to smoothly engage the outer circumferential helical teeth of the upper punch with the inner circumferential helical teeth of the floating die, the outer circumferential helical teeth of the upper punch are adapted to engage with inner circumferential helical teeth of a phase guide for phase adjustment.
In such a method of manufacturing helical gears by compacting powdered materials as described above, when a helix angle of the helical teeth is deep or the teeth profile is thin, there is an inconvenience that cracks will be caused in a part of the teeth, especially when the floating die, which has held the periphery of the compacted product in position, is moved downward after powdered materials have been compacted and released from the compacted product.
It is, therefore, an object of the present invention to prevent cracks from being caused especially when an intermediary die, which has held the periphery of a compacted product in position, is released, upon compacting powder materials.
To accomplish the above-mentioned object, according to the present invention, a method of manufacturing helical gears by compacting powdered materials comprises the steps of engaging an intermediary die provided with inner circumferential helical teeth with a lower punch provided with outer circumferential helical teeth, filling the powdered materials into a concave area formed by the lower punch and the intermediary die, moving an upper punch which is provided with outer circumferential helical teeth downward along a phase guide to engage with the intermediary die, wherein pressure is applied to the powdered materials to compact them, holding a compacted product with a predetermined holding power between the upper and lower punches, then releasing the intermediary die downward from the compacted product, wherein lateral displacement which is caused on the phase guide when the intermediary die is released, is forcibly corrected to allow the guide to return to its original position.
As a result of investigating the crack-generating mechanism, inventors of the present invention found that the cracks were caused by prizing of the upper surface of the compacted product by the upper punch when the intermediary die which holds the periphery of a compacted product in position is released while the compacted product is pressed with a predetermined holding power by the upper punch.
Namely, the upper punch is arranged to apply a pressure of about 420 tons when the powdered materials are compacted, but since the upper punch corresponding to a part of the helical teeth inclines relative to its vertical direction, it applies lateral load on a part of the intermediary die. After the powder materials are compacted, when the pressure from the upper punch is reduced to provide a holding power of about 4 tons, and the intermediary die is released while holding the compacted product, both the compacted product, held by the intermediary die in position until then, and the upper punch are now kept free. As a result, relative lateral displacement is caused between the upper punch and the compacted product to produce slide contact force with respect to the contacting portions therebetween. With this slide contact force, it is thought that large prizing force is developed on the upper surface of the compacted product to cause the cracks.
One the other hand, as described above, the upper punch is guided by a phase adjustment guide when it engages with the inner circumferential teeth of the intermediary die. Thus, when the upper punch applies a pressure of about 420 tons, it is considered that this phase guide will also cause lateral displacement (phase displacement) therein. Relative displacement, which is caused between the upper punch and the compacted product when the intermediary die is released, is also caused by this lateral displacement (phase displacement) of this phase guide, which is one of the main causes of the cracks.
Thus, by forcibly correcting the lateral displacement (phase displacement) which occurs at the phase guide when the intermediary die is released, the upper punch is positioned to return to its original position so that the prizing force developed on the compacted product can be reduced.
Herein, a concrete means for correcting the lateral displacement of the phase guide may be freely chosen, but it is possible to use a correcting means such as wedge action and multiple action.
In another aspect of the present invention, a method of manufacturing helical gears by compacting powdered materials comprises the steps of engaging an intermediary die provided with inner circumferential helical teeth with a lower punch provided with outer circumferential helical teeth; filling powdered materials into a concave area which is formed by the lower punch and the intermediary die; moving the upper punch provided with outer circumferential helical teeth downward along a phase guide so as to engage with the intermediary die, wherein the powdered materials are compacted by applying pressing load; holding the compacted product, with a predetermined holding power, between the upper and lower punches; then releasing the intermediary die downward from the compacted product, wherein the upper punch is provided, at its pressing surface, with an escape surface which serves to reduce slide contact force of the upper punch produced with respect to the compacted product when the intermediary die is released and relative lateral displacement is caused to occur between the upper punch and the compacted product.
Namely, the pressing surface of the conventional upper punch is formed flat and when the relative lateral displacement is caused to occur between the compacted product and the upper punch, this corresponds with the direction of the contact surface and is greatly influenced by the slide contact force. Therefore, if the pressing surface of the upper punch is provided with the escape surface which inclines in such a direction that both contact surfaces part when the relative lateral displacement occurs, it is possible to reduce the slide contact force of the upper punch against the compacted product.
In still another aspect of the present invention, the method of correcting the lateral displacement of the phase guide and the escape surface of the upper punch as described above are combined.
With this combination, it is possible to effectively prevent the prizing.